Your son is increasing your milk supply by frequent eating. This is natural. He is probably going through a growth spurt.Do not supplement with formula or baby food. Let the baby do his work to increase your supply by eating. DO NOT USE THE PUMP TO ASSESS YOUR MILK SUPPLY! Your baby can get much more out of you than a pump. Pumping takes practice. Many people can't get much milk with the pump, but their babies can get the milk.
Also, you do not need to feel engorged to have adequate milk supply. You are at the point where your body is adjusting to your baby. The time of the greatest milk production is DURING nursing, so you don't need to let your breasts "fill up" before you feed your baby. Just put baby to breast when he is hungry and he will get milk.
Check out this link from Kelly Mom:
http://www.kellymom.com/bf/supply/low-supply.html
The biggest indicator of if you have enough milk is if you are getting out put from the baby: wet and dirty diapers and if the baby is gaining weight. But, both of these can be tricky as well. Some babies just don't poop as often as others. You should be getting a good number of wet diapers, though. Also, on the weight, formula fed babies are often fatter than breastfed babies. This does not mean they are healthier. You should see a steady weight gain, but it does slow after the first few months. Your baby should not be losing weight, though.
If there is a lactation consultant around or a La Leche League group, they can provide in-person reassurance.
Hang in there! Nursing is great, especially as the child gets older. You are so much more mobile and flexible, especially if you learn to nurse your baby in a carrier like the Ergo http://www.ergobaby.com
or some sort of sling or wrap. If your baby is fussing at the grocery store, you put him in the carrier and nurse. Most people will just think he is sleeping. I've nursed while hiking up a mountain in the Egyptian desert, while switching terminals during international travel and everywhere in between.